Pulsed-beam atomic magnetometer system

ABSTRACT

One example includes a magnetometer system. The system includes a sensor cell comprising alkali metal vapor and a laser system configured to provide an optical pump beam through the sensor cell in a pulsed manner to facilitate precession of the alkali metal vapor in response to an external magnetic field and to provide an optical probe beam through the sensor cell in a pulsed manner based on a precession frequency of the alkali metal vapor. The system also includes a detection system configured to detect the precession of the alkali metal vapor in response to a detection beam corresponding to the optical probe beam exiting the sensor cell and to calculate an amplitude and direction of the external magnetic field based on the detected precession of the alkali metal vapor.

RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application Ser.No. 62/513,069, filed 31 May 2017, which is incorporated herein in itsentirety.

TECHNICAL FIELD

This disclosure relates generally to sensor systems, and morespecifically to a pulsed-beam atomic magnetometer.

BACKGROUND

Magnetometer systems, such as nuclear magnetic resonance (NMR)magnetometers and/or electron paramagnetic resonance (EPR)magnetometers, can include a cell that contains one or more alkali metalvapors, such as rubidium or cesium, which can exhibit precessioncharacteristics that can be a function of an external magnetic field.Thus, the magnetometer system can be configured to detect the externalmagnetic field based on the precession characteristics of the alkalimetal vapor(s). Typical magnetometer systems that implement detection ofthe external magnetic field in three vector axes implement a combinationof multiple single-axis or dual-axis vector systems. Such magnetometersystems can typically exhibit sensitivities to dynamics or systemmisalignments when attempting to determine a whole field scalarmeasurement, which can result in inaccuracy. Thus, when high sensitivityand stability may be required in a dynamic environment, whole fieldscalar magnetometer systems are often implemented.

SUMMARY

One example includes a magnetometer system. The system includes a sensorcell comprising alkali metal vapor and a laser system configured toprovide an optical pump beam through the sensor cell in a pulsed mannerto facilitate precession of the alkali metal vapor in response to anexternal magnetic field and to provide an optical probe beam through thesensor cell in a pulsed manner based on a precession frequency of thealkali metal vapor. The system also includes a detection systemconfigured to detect the precession of the alkali metal vapor inresponse to a detection beam corresponding to the optical probe beamexiting the sensor cell and to calculate an amplitude and direction ofthe external magnetic field based on the detected precession of thealkali metal vapor.

Another example includes a method for measuring an external magneticfield via a magnetometer system. The method includes generating acircularly-polarized optical pump beam via a pump laser and generating alinearly-polarized optical probe beam via a probe laser. The method alsoincludes providing the circularly-polarized optical pump beam through asensor cell comprising alkali metal vapor in a pulsed-manner based on atiming signal to facilitate precession of the alkali metal vapor inresponse to an external magnetic field. The method also includesproviding the linearly-polarized optical probe beam through the sensorcell in a pulsed-manner based on the timing signal to provide adetection beam corresponding to the linearly-polarized optical probebeam exiting the sensor cell. The method also includes detecting theprecession of the alkali metal vapor based on the detection beam andgenerating the timing signal based on the detected precession of thealkali metal vapor. The method further includes calculating an amplitudeand direction of the external magnetic field based on the detectedprecession of the alkali metal vapor.

Another example includes a magnetometer system. The system includes asensor cell comprising alkali metal vapor and comprising a firstmeasurement zone, a second measurement zone, and a third measurementzone. The second and third measurement zones can be arranged at oppositeends of the sensor cell. The system also includes a laser systemconfigured to provide an optical pump beam through each of the first,second, and third measurement zones of the sensor cell in a pulsedmanner to facilitate precession of the alkali metal vapor in response toan external magnetic field and to provide an optical probe beam througheach of the first, second, and third measurement zones of the sensorcell in a pulsed manner based on a precession frequency of the alkalimetal vapor. The system further includes a detection system configuredto detect the precession of the alkali metal vapor in response to afirst detection beam, a second detection beam, and a third detectionbeam corresponding to the optical probe beam exiting the sensor cellthrough the first, second, and third measurement zones, respectively, tocalculate a scalar amplitude and direction of the external magneticfield in response to the first detection beam, and to calculate amagnetic field gradient of the external magnetic field in response tothe second and third detection beams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a magnetometer system.

FIG. 2 illustrates an example diagram of an external magnetic fieldthrough a sensor cell.

FIG. 3 illustrates an example diagram of interrogation of a sensor cell.

FIG. 4 illustrates another example diagram of an external magnetic fieldthrough a sensor cell.

FIG. 5 illustrates an example of a timing diagram.

FIG. 6 illustrates another example of a magnetometer system.

FIG. 7 illustrates an example of a method for measuring an externalmagnetic field.

DETAILED DESCRIPTION

This disclosure relates generally to sensor systems, and morespecifically to a pulsed-beam atomic magnetometer. The magnetometersystem can be configured as a Synchronous Light-pulse AtomicMagnetometer (SLAM) system that includes a concurrent pump/pulse beampumping and interrogation methodology, such as similar to Bell-Bloomall-optical magnetometry. The magnetometer system includes a lasersystem that includes at least one pump laser and at least one probelaser configured to generate a respective at least one optical pump beamand a respective at least one optical probe beam. As an example, thepump and optical probe beam(s) can be combined via an optical combiner(e.g., a 2×2 optical combiner) to provide the pump and optical probebeam(s) in a collinear manner. The pump and optical probe beam(s) areprovided through a sensor cell that includes an alkali metal vapor. Thealkali metal vapor can precess in response to an external magnetic fieldbased on alignment of the net magnetic moments of the alkali metal vaporin the cell in response to the circularly-polarized optical pump beam.As a result of the precession, the linearly-polarized optical probe beamcan experience a Faraday rotation as it passes through the sensor cell,with the Faraday rotation being based on the instantaneous orientationof the net magnetic moments of the alkali metal vapor as the alkalimetal vapor precesses. Therefore, detection beam(s) corresponding to theoptical probe beam(s) exiting the sensor cell can be monitored tomonitor the precession of the alkali metal vapor.

As an example, the monitored precession of the alkali metal vapor can beprovided as feedback to a timing controller to generate a timing signal.The timing signal can thus be provided to the laser system to provide atiming reference as to when to provide the pulsed optical pump beam andoptical probe beam through the sensor cell. For example, the lasersystem can provide an optical pump beam pulse through the sensor cell inresponse to the timing signal to pump the alkali metal vapor once eachperiod of the precession (e.g., when the magnetic moments are alignedapproximately parallel with the optical pump beam axis). As anotherexample, the laser system can provide an optical probe beam pulsethrough the sensor cell in response to the timing signal when themagnetic moments of the alkali metal vapor are approximately paralleland anti-parallel with the optical probe beam axis to calibrate themagnetometer system. Furthermore, the laser system can provide anoptical probe beam pulse through the sensor cell in response to thetiming signal when the magnetic moments of the alkali metal vapor areapproximately orthogonal with the optical probe beam axis to monitor theamplitude and direction of the external magnetic field, as indicated bythe Faraday rotation of the linear polarization.

Furthermore, the sensor cell can include three distinct measurementzones through which the optical pump beam and optical probe beam can beprovided. The first measurement zone can be through an approximatecenter of the sensor cell, such that the timing signal can be generatedbased on the respective detection beam that is provided from the firstmeasurement zone. The detection beam provided from the first measurementzone can also determine a scalar amplitude and direction of the externalmagnetic field. In addition, the second and third measurement zones canbe arranged at regions of substantially opposite ends of the sensorcell. As a result, a second detection beam corresponding to the opticalprobe beam exiting the sensor cell through the second measurement zoneand a third detection beam corresponding to the optical probe beamexiting the sensor cell through the third measurement zone can beimplemented to determine a magnetic field gradient of the externalmagnetic field based on a differential measurement of the magnetic fieldthrough each of the second and third measurement zones.

FIG. 1 illustrates an example of a magnetometer system 10. Themagnetometer system 10 can be implemented in any of a variety ofapplications to measure a magnetic field, such as navigation. Forexample, the magnetometer system 10 can be implemented in an inertialnavigation system (INS) for an aircraft or a spacecraft to assist withreal-time navigation or location determination.

The magnetometer system 10 includes a laser system 12 that includes atleast one pump laser 14 and at least one probe laser 16. The pumplaser(s) 14 are each configured to generate a respective optical pumpbeam OPT_(PMP), and the probe laser(s) 16 are each configured togenerate a respective optical probe beam OPT_(PRB). The optical pumpbeam(s) OPT_(PMP) and the optical probe beam(s) OPT_(PRB) are eachprovided through a sensor cell 18 that includes an alkali metal vapordisposed therein. In the example of FIG. 1, the sensor cell 18 includesa first measurement zone 20 (“ZONE 1”), a second measurement zone 22(“ZONE 2”), and a third measurement zone 24 (“ZONE 3”) that can eachcorrespond to three-dimensional spatial regions within the volume of thesensor cell 18. As an example, the first measurement zone 20 can bearranged approximately centrally along a length of the sensor cell 18,and the second and third measurement zones 22 and 24 can be arranged atopposing ends of the sensor cell 18. As described in greater detailherein, the first, second, and third measurement zones 20, 22, and 24can be implemented for calibration of the magnetometer system 10, fordetermining the amplitude and direction of an external magnetic field,for determining a magnetic field gradient associated with the externalmagnetometer, and for feedback to generate a timing signal associatedwith the precession of the alkali metal vapor.

The optical pump beam OPT_(PMP) can be provided through the sensor cell18 to facilitate precession of the alkali metal vapor in response to theexternal magnetic field. As an example, the optical pump beam OPT_(PMP)can be circularly-polarized, such that the angular momentum of thephotons of the optical pump beam OPT_(PMP) can be absorbed by the alkalimetal vapor (e.g., based on the wavelength of the optical pump beamOPT_(PMP)). Therefore, the optical pump beam OPT_(PMP) can align themagnetic moment of the alkali metal vapor in an approximately parallelmanner with respect to the optical pump beam OPT_(PMP). Therefore, thealkali metal vapor can precess about the external magnetic field basedon the alignment of the magnetic moment of the alkali metal vapor.

FIG. 2 illustrates an example diagram 50 of an external magnetic fieldB_(EXT) through a sensor cell 52. In the example of FIG. 2, the sensorcell is demonstrated in a first view 54 and a second view 56 that areorthogonal with respect to each other. Particularly, the first view 54is demonstrated from a view along the Z-axis based on Cartesiancoordinate system 58, and the second view 56 is demonstrated from a viewalong the Y-axis based on Cartesian coordinate system 60. The sensorcell 52 can correspond to the sensor cell 18 in the example of FIG. 1.Therefore, reference is to be made to the example of FIG. 1 in thefollowing description of the example of FIG. 2.

In the example of FIG. 2, an external magnetic field B_(EXT) isdemonstrated as being provided orthogonally through the sensor cell 52in the −Z direction. The sensor cell 52 is also demonstrated asincluding the alkali metal vapor arranged as having a magnetic momentvector B_(MM) that extends along the X-axis. The magnetic moment vectorB_(MM) can correspond to a parallel arrangement of the magnetic momentof the alkali metal vapor in response to being pumped by the opticalpump beam OPT_(PMP). As an example, the optical pump beam OPT_(PMP) canbe periodically provided in a pulsed manner to periodically align themagnetic moment vector B_(MM) parallel (e.g., collinear and in the samedirection) with the optical pump beam OPT_(PMP).

In response to the external magnetic field B_(EXT) through the sensorcell 52, the magnetic moment of the alkali metal vapor can precess aboutthe external magnetic field B_(EXT). In the example of FIG. 2, becausethe external magnetic field B_(EXT) is provided orthogonally through thesensor cell 52, and thus orthogonally with respect to the magneticmoment vector B_(MM), the magnetic moment vector B_(MM) can precessclockwise in the XY-plane, and thus orthogonally with respect to theexternal magnetic field B_(EXT). The amplitude of the external magneticfield B_(EXT) can determine the frequency of precession of the magneticmoment vector B_(MM). In addition, as described in greater detailherein, the angle of the external magnetic field B_(EXT) with respect tothe angle of the magnetic moment vector B_(MM) can affect the angle ofthe magnetic moment vector B_(MM) during a period of precession. Theangle of the magnetic moment vector B_(MM) can thus be detected by themagnetometer system, as described in greater detail herein, and can beindicative of the angle of the external magnetic field B_(EXT).

Referring back to the example of FIG. 1, the magnetometer system 10 canalso include a detection system 26. The detection system 26 isconfigured to monitor at least one detection beam OPT_(DET) that isprovided from the sensor cell 18. As an example, the detection beam(s)OPT_(DET) can correspond to the optical probe beam OPT_(PRB) beingprovided through at least one of the respective measurement zones 20,22, and 24 and exiting the sensor cell 18. As described previously, theoptical probe beam OPT_(PRB) can be provided as linearly-polarized.Therefore, based on the precession of the magnetic moment vector B_(MM)of the alkali metal vapor, the optical probe beam OPT_(PRB) canexperience Faraday rotation, such that the respective detection beam(s)OPT_(DET) can exhibit the Faraday rotation that can be indicative of theprecession of the alkali metal vapor based on the external magneticfield B_(EXT).

In the example of FIG. 1, the detection system 26 includes at least oneoptical detector 28 that is configured to detect the Faraday rotation ofthe respective detection beam(s) OPT_(DET) to monitor the precession ofthe alkali metal vapor in response to the external magnetic fieldB_(EXT). As an example, the optical detector(s) 28 can be configured asan polarization beamsplitter and set of photodetectors (e.g.,photodiodes) that can measure a relative Faraday rotation of thedetection beam(s) OPT_(DET). Therefore, based on the measured Faradayrotation of the detection beam(s) OPT_(DET), the detection system 26 cancalculate an amplitude and direction of the external magnetic fieldB_(EXT), as described in greater detail herein. In the example of FIG.1, the detection system 26 is demonstrated as calculating a scalaramplitude and direction of the external magnetic field B_(EXT),demonstrated as a signal B_(SCLR), and as calculating a magnetic fieldgradient of the external magnetic field B_(EXT), demonstrated as asignal B_(GRDT).

In the example of FIG. 1, the detection system 26 also includes a timingcontroller 30. As described previously, the laser system 12 can providethe optical pump beam OPT_(PMP) via the pump laser 14 in a pulsedmanner, and can likewise provide the optical probe beam OPT_(PRB) viathe probe laser 16 in a pulsed manner. The laser system 12 can providethe pulses of the optical pump beam OPT_(PMP) and the optical probe beamOPT_(PRB) in response to a timing signal TMR that is generated by thetiming controller 30 in response to the detection beam(s) OPT_(DET). Forexample, the timing signal TMR can correspond to the period of theprecession of the alkali metal vapor, such that the timing signal TMRcan provide an indication to the laser system 12 as to the time toactivate the pump laser(s) 14 to provide the pulse(s) of the opticalpump beam OPT_(PMP) and as to the time to activate the probe laser(s) 16to provide the pulse(s) of the optical probe beam OPT_(PRB).

FIG. 3 illustrates an example diagram 100 of interrogation of a sensorcell 102. In the example of FIG. 3, the diagram 100 includes threeseparate states of the sensor cell 102, demonstrated as a first state104, a second state 106, and a third state 108. The sensor cell 102 cancorrespond to the sensor cell 18 in the example of FIG. 1, and thesensor cell 52 in the example of FIG. 2. As an example, the sensor cell102 can correspond to a given one of the measurement zones 20, 22, and24 in the sensor cell 18 demonstrated in the example of FIG. 1.Therefore, reference is to be made to the example of FIGS. 1 and 2 inthe following description of the example of FIG. 3.

Each of the states 104, 106, and 108 of the sensor cell 102 correspondto separate respective orientations of the magnetic moment vector B_(MM)of the alkali metal vapor during a portion of the precession of thealkali metal vapor in response to the external magnetic field B_(EXT)(not shown in the example of FIG. 3). The first state 104 demonstratesthe magnetic moment vector B_(MM) extending in the +X direction based onthe Cartesian coordinate system 110, which can correspond to a directionparallel to the optical probe beam OPT_(PRB). As an example, the opticalprobe beam OPT_(PRB) and the optical pump beam OPT_(PMP) can becollinear, such that the first state 104 can demonstrate the magneticmoment vector B_(MM) extending parallel with both the optical probe beamOPT_(PRB) and the optical pump beam OPT_(PMP). As an example, the firststate 104 can correspond to a beginning of a precession period of themagnetic moment vector B_(MM), such that the timing signal TMR cancommand the laser system 12 to provide a pulse of the optical pump beamOPT_(PMP) via the pump laser 14 to align the alkali metal vapor to formthe magnetic moment vector B_(MM).

In the first state 104, the optical probe beam OPT_(PRB) is demonstratedas being provided through the sensor cell 102 to provide a detectionbeam OPT_(DET) exiting the sensor cell 102 and being provided to anoptical detector 112. As an example, the optical detector 112 cancorrespond to an optical detector 28 in the example of FIG. 1. Theoptical detector 112 includes a polarization beamsplitter 114, a firstphotodetector (“PD1”) 116, and a second photodetector (“PD2”) 118. Inthe example of FIG. 3, the detection beam OPT_(DET) can exhibit asubstantial maximum Faraday rotation in a first direction about themagnetic moment vector B_(MM) based on the optical probe beam OPT_(PRB)being provided through the sensor cell 102 parallel with the magneticmoment vector B_(MM). As a result, the detection beam OPT_(DET) can passpreferentially through the polarization beamsplitter 114 to the firstphotodetector 116. As a result, the detection system 26 in the exampleof FIG. 1 can identify an approximate magnitude of the magnetic momentvector B_(MM) at the beginning of a precession period of the precessionof the alkali metal vapor. For example, the timing signal TMR cancommand the laser system 12 to provide a calibration pulse of theoptical probe beam OPT_(PRB) via the probe laser 16 to determine anapproximate magnitude of the magnetic moment vector B_(MM) and todetermine if the magnetic moment vector B_(MM) is parallel with theoptical probe beam OPT_(PRB).

The second state 106 demonstrates the magnetic moment vector B_(MM)extending in the −X direction based on the Cartesian coordinate system110, which can correspond to a direction anti-parallel to the opticalprobe beam OPT_(PRB). As an example, the second state 106 can correspondto an approximately 180° phase of the precession period of the magneticmoment vector B_(MM). In the second state 106, the optical probe beamOPT_(PRB) is demonstrated as being provided through the sensor cell 102to provide a detection beam OPT_(DET) exiting the sensor cell 102 andbeing provided to the optical detector 112. In the example of FIG. 3,the detection beam OPT_(DET) can exhibit an approximate maximum Faradayrotation in a second direction opposite the first direction based on theoptical probe beam OPT_(PRB) being provided through the sensor cell 102anti-parallel with the magnetic moment vector B_(MM). As a result, thedetection beam OPT_(DET) can be preferentially reflected by thepolarization beamsplitter 114 to the second photodetector 118. As aresult, the detection system 26 in the example of FIG. 1 can identifythat the magnetic moment vector B_(MM) is at the time during theprecession period of the precession of the alkali metal vapor that themagnetic moment vector B_(MM) is anti-parallel with respect to theoptical probe beam OPT_(PRB) and can determine an approximate magnitudeof the magnetic moment vector B_(MM) at the 180° phase in the precessioncycle. For example, the timing signal TMR can command the laser system12 to provide a calibration pulse of the optical probe beam OPT_(PRB)via the probe laser 16 to determine if the magnetic moment vector B_(MM)is anti-parallel with the optical probe beam OPT_(PRB) and measure itsmagnitude.

The third state 108 demonstrates the magnetic moment vector B_(MM)extending in either the +Y direction or −Y direction based on theCartesian coordinate system 110, which can correspond to directionsorthogonal to the optical probe beam OPT_(PRB). As an example, the thirdstate 106 can correspond to either approximately 90° phase or 270° phaseof the precession period of the magnetic moment vector B_(MM). In thethird state 108, the optical probe beam OPT_(PRB) is demonstrated asbeing provided through the sensor cell 102 to provide a detection beamOPT_(DET) exiting the sensor cell 102 and being provided to the opticaldetector 112. In the example of FIG. 3, the detection beam OPT_(DET) canexhibit an approximate zero Faraday rotation based on the optical probebeam OPT_(PRB) being provided through the sensor cell 102 orthogonallywith the magnetic moment vector B_(MM). As a result, the detection beamOPT_(DET) can be partially (e.g., half) reflected by the polarizationbeamsplitter 114, and thus provided approximately equally to each of thefirst photodetector 116 and the second photodetector 118. As a result,the detection system 26 in the example of FIG. 1 can identify that themagnetic moment vector B_(MM) is at the time during the precessionperiod of the precession of the alkali metal vapor that the magneticmoment vector B_(MM) is approximately orthogonal with respect to theoptical probe beam OPT_(PRB). For example, the timing signal TMR cancommand the laser system 12 to provide a measurement pulse of theoptical probe beam OPT_(PRB) via the probe laser 16 to determine if themagnetic moment vector B_(MM) is orthogonal to the optical probe beamOPT_(PRB).

Therefore, based on the states 104, 106, and 108 demonstrated in theexample of FIG. 3, the detection system 26 can monitor the detectionbeam OPT_(DET) at the appropriate times in a feedback manner based onthe timing signal TMR to monitor the precession of the alkali metalvapor, and thus to determine the amplitude and direction of the externalmagnetic field B_(EXT). As an example, upon initialization of themagnetometer system 10, the detection system 26 can command the lasersystem 12 to provide the optical probe beam OPT_(PRB) substantiallycontinuously through the first measurement zone 20 of the sensor cell 18to determine an initial precession of the alkali metal vapor in thesensor cell 18 in response to the external magnetic field B_(EXT). Forexample, the detection system 26 can generate a waveform based on adifferential measurement between the first photodetector 116 and thesecond photodetector 118 to determine an initial frequency of theprecession of the external magnetic field B_(EXT), which can correspondto the amplitude of the external magnetic field B_(EXT). In addition,the initial precession can also indicate the portions of the precessionthat correspond to the anti-parallel orientation of the magnetic momentof the alkali metal vapor, such as demonstrated in the second state 106in the example of FIG. 3, and the portions of the precession thatcorrespond to the orthogonal orientations of the magnetic moment of thealkali metal vapor, such as demonstrated in the third state 108 in theexample of FIG. 3. The relative offset of the timing of the second andthird states 106 and 108, respectively, can be indicative of the angleof the external magnetic field B_(EXT), as described in greater detailherein.

In response to determining the initial precession of the alkali metalvapor, the timing controller 30 can generate the timing signal TMR basedon the initial precession of the alkali metal vapor. The detectionsystem 26 can thus cease the substantially constant application of theoptical probe beam OPT_(PRB), such that the laser system 12 can providethe optical pump beam OPT_(PMP) pulses via the pump laser 14 and theoptical probe beam OPT_(PRB) pulses via the probe laser 16 based on thetiming signal TMR. As a result, the detection system 26 can monitor thedetection beam OPT_(DET) from the first measurement zone 20 to monitorthe precession of the alkali metal vapor based on observing thedetection beam OPT_(DET) in the third state 108. As an example, inresponse to detecting an uneven amplitude of the detection beamOPT_(DET) with respect to the first and second photodetectors 116 and118, the detection system 26 can identify that the external magneticfield B_(EXT) has changed in amplitude and/or direction based on theprecession of the alkali metal vapor being misaligned from theapplication of the optical probe beam OPT_(PRB) pulse in response to thetiming signal TMR. Therefore, the detection system 26 can identify thechange in the external magnetic field B_(EXT) (e.g., providing themeasured magnetic field B_(SCLR) appropriately), and the timingcontroller 30 can change the timing signal TMR accordingly.

As described previously, the first state 104 can correspond to thealignment of the magnetic moment vector B_(MM) in response to theoptical pump beam OPT_(PMP) pulse. However, as also describedpreviously, the precession of the magnetic moment of the alkali metalvapor is provided about the direction of the external magnetic fieldB_(EXT). Therefore, the precession of the alkali metal vapor may notnecessarily be planar with respect to the XY-plane, as demonstrated inthe examples of FIGS. 2 and 3. Therefore, the relative timing of thesecond and third states 106 and 108 may not necessarily correspond to180° of the precession period for the second state and 90° and 270° ofthe precession period for the third state 108.

FIG. 4 illustrates another example diagram 150 of an external magneticfield B_(EXT) through a sensor cell 152. In the example of FIG. 4, thesensor cell is demonstrated in a first view 154 and a second view 156that are orthogonal with respect to each other. Particularly, each ofthe first view 154 and second view 156 are demonstrated from a viewalong the Y-axis based on Cartesian coordinate system 158. The sensorcell 152 can correspond to the sensor cell 18 in the example of FIG. 1.Therefore, reference is to be made to the example of FIGS. 1-3 in thefollowing description of the example of FIG. 4.

The sensor cell 152 is demonstrated as including the alkali metal vaporarranged as having a magnetic moment vector B_(MM) that extends alongthe X-axis. Similar to as described previously, the magnetic momentvector B_(MM) can correspond to a parallel arrangement of the magneticmoment of the alkali metal vapor in response to being pumped by theoptical pump beam OPT_(PMP). As an example, the optical pump beamOPT_(PMP) can be periodically provided in a pulsed manner toperiodically align the magnetic moment vector B_(MM) parallel (e.g.,collinear and in the same direction) with the optical pump beamOPT_(PMP). In the example of FIG. 4, an external magnetic field B_(EXT)is demonstrated as being provided through the sensor cell 152 at anangle θ₁ relative to the −Z axis, and thus provides an angle of θ₂relative to the magnetic moment vector B_(MM), wherein θ₂ minus θ₁ isequal to approximately 90°.

In response to the external magnetic field B_(EXT) through the sensorcell 152, the magnetic moment of the alkali metal vapor can precessabout the external magnetic field B_(EXT). In the example of FIG. 4,because the external magnetic field B_(EXT) is provided at the angle θ₁through the sensor cell 152, the magnetic moment vector B_(MM) canprecess at the angle θ₂ about the external magnetic field B_(EXT). Inthe example of FIG. 2, the second view 156 can thus demonstrate themagnetic moment vector B_(MM) at a portion of the precession that isopposite the portion of the precession demonstrated in the first view154 (e.g., at 180° out-of-phase during the precession of the magneticmoment vector B_(MM)). In addition, it is to be understood that themagnetic moment vector B_(MM) will not be collinear with the Y-axisduring the 90° and 270° phases of the precession of the alkali metalvapor, but will instead be angularly offset from the Y-axis based on theangle θ₁ of the external magnetic field B_(EXT) relative to the Z-axis.As a result, the measured amplitude of the external magnetic fieldB_(EXT) will not be symmetrical about zero at each 180° phase intervalof the precession period of the alkali metal vapor, as described ingreater detail herein with reference to FIG. 5.

FIG. 5 illustrates an example of timing diagrams 200 and 202. The timingdiagram 200 can correspond to a precession of the alkali metal vaporbased on the external magnetic field B_(EXT) being provided orthogonallywith respect to the axis of the optical probe beam OPT_(PRB), and thusalong the −Z axis, as demonstrated in the example of FIG. 2. The timingdiagram 200 demonstrates the spin projection of the magnetic momentvector B_(MM) along the optical probe beam OPT_(PRB) as a solid line.The timing diagram 200 further demonstrates the relative sensitivity ofmeasurement of gradients in the amplitude of the external magnetic fieldB_(EXT) as a function of the direction of the magnetic moment vectorB_(MM), and thus based on the optical detector(s) 28 as a function ofthe Faraday rotation of the detection beam(s) OPT_(DET), as a dottedline.

Similarly, the timing diagram 202 can correspond to a precession of thealkali metal vapor based on the external magnetic field B_(EXT) beingprovided at an angle θ₁=45° with respect to the axis of the optical pumpbeam OPT_(PMP), and thus such that the angle θ₂=45°, similar to asdemonstrated in the example of FIG. 4 (whereas the angles θ₁ and θ₂ arenot necessarily illustrated to scale). The timing diagram 202demonstrates the spin projection of the magnetic moment vector B_(MM)along the optical probe beam OPT_(PRB) as a solid line. The timingdiagram 202 further demonstrates the relative sensitivity of measurementof gradients in the amplitude of the external magnetic field B_(EXT) asa function of the magnetic moment vector B_(MM), and thus based on theoptical detector(s) 28 as a function of the Faraday rotation of thedetection beam(s) OPT_(DET), as a dotted line.

In the example of FIG. 5, the periods of the precession of the magneticmoment vector B_(MM) are demonstrated at intervals 204, beginning at atime T₀. Particularly, at a time T₀ of each precession period, the lasersystem 12 can provide a pulse of the optical pump beam OPT_(PMP) toalign the magnetic moment vector B_(MM) along the axis of the opticalpump beam OPT_(PMP). Subsequently, the magnetic moment vector B_(MM)precesses about the external magnetic field B_(EXT). A time T₁corresponds to 90° of the precession period, a time T₂ corresponds to180° of the precession period, a time T₃ corresponds to 270° of theprecession period, and a time T₄ corresponds to completion of theprecession period and the beginning of a next precession period. Thus,at the time T₄, the laser system 12 can again provide a pulse of theoptical pump beam OPT_(PMP) to align the magnetic moment vector B_(MM)along the axis of the optical pump beam OPT_(PMP).

In addition, similar to as described previously, the laser system 12 canprovide pulses of the optical probe beam OPT_(PRB) to calibrate themagnetometer system 10. As an example, the laser system 12 can providepulses of the optical probe beam OPT_(PRB) at the time T₀ and the timeT₂, and thus at 180° opposite phases of the precession of the alkalimetal vapor, to calibrate the magnetometer system 10. As an example, thecalibration pulses of the optical probe beam OPT_(PRB) can correspond tothe timing of the first state 104 and the second state 106 demonstratedin the example of FIG. 3.

For example, the calibration pulses of the optical probe beam OPT_(PRB)can be substantially reduced in optical energy relative to interrogationpulses intended to measure the amplitude of the external magnetic fieldB_(EXT) and gradients therein, as described in greater detail herein.Therefore, in response to the calibration pulse provided at the timeT₀/T₄, the optical detector(s) 112 can determine that the optical energyof the detection beam OPT_(DET) is provided preferentially on either thefirst photodetector 116 or the second photodetector 118. Similarly, inresponse to the calibration pulse provided at the time T₂, the opticaldetector(s) 112 can determine that the optical energy of the detectionbeam OPT_(DET) is provided preferentially on the other of the firstphotodetector 116 and the second photodetector 118. The laser system 12can also provide measurement pulses of the optical probe beam OPT_(PRB)at times T₁ and T₃ in the example of timing diagram 200 or times T₂, T₅and T₆ in the example of timing diagram 202. These measurement pulsescan be of a substantially higher magnitude than the optical probe beamOPT_(PRB) pulses provided during times T₀/T₄ and T₂. If the detectionsystem 26 determines that the optical energy of detection beam OPT_(DET)is shared unequally between the first and second photodetectors 116 and118 during a measurement pulse of the optical probe beam OPT_(PRB) attime T₁ or T₃ in the example of timing diagram 200 or T₂ in the exampleof timing diagram 202, the timing controller 28 can adjust the timingsignal TMR accordingly in a feedback manner. Changes to the sharing ofoptical energy of the detection beam OPT_(DET) on the photodetectors 116and 118 can indicate a change in amplitude of the external magneticfield B_(EXT), given that the amplitude of the external magnetic fieldB_(EXT) affects the frequency of the precession of the alkali metalvapor, and thus the length of the precession period of the alkali metalvapor.

The timing diagram 200 thus demonstrates a sinusoidal spin projection ofthe magnetic moment vector B_(MM) about zero based on the planarprecession of the magnetic moment vector B_(MM), as demonstrated in theexample of FIG. 2. Additionally, because the precession of the magneticmoment vector B_(MM) is planar in the example of FIG. 2 based on theexternal magnetic field B_(EXT) being orthogonal to the axis of theoptical probe beam OPT_(PRB), the external magnetic field B_(EXT), asobserved by the optical detector(s) 28 via the detection beam(s)OPT_(DET), is symmetrical about zero at each 180° interval of the periodof the precession. The laser system 12 can be configured to provide thepulses of the optical probe beam OPT_(PRB) at the times corresponding toan approximately equal optical energy of the detection beam OPT_(DET) isprovided on the first and second photodetectors 116 and 118 of theoptical detector(s) 112 to determine the angle of the external magneticfield B_(EXT).

For the timing diagram 200, in which the external magnetic field B_(EXT)is provided orthogonally with respect to the optical axis of the opticalprobe beam OPT_(PRB), the time during the precession of the alkali metalvapor at which approximately equal optical energy of the detection beamOPT_(DET) is provided on the first and second photodetectors 116 and 118of the optical detector(s) 112 corresponds to third state 108 in theexample of FIG. 3 (e.g., at both 90° and 270°). Therefore, the lasersystem 12 can provide the optical probe beam OPT_(PRB) pulses at thetimes T₁ and T₃ in the timing diagram 200 to monitor the magnitude andangle of the external magnetic field B_(EXT). Deviations of the angleand/or amplitude of the external magnetic field B_(EXT) can thus beobserved by the detection system 26 in response to the optical energy ofthe detection beam OPT_(DET) provided on the first and secondphotodetectors 116 and 118 of the optical detector(s) 112 being unequal,as described in greater detail in the timing diagram 202.

Similar to the timing diagram 200, the timing diagram 202 demonstrates asinusoidal spin projection of the magnetic moment vector B_(MM).However, because of the angle θ₁=45°, the spin projection of themagnetic moment vector B_(MM) along the direction of the optical probebeam OPT_(PRB) is tangent to zero at the time T₂ in the timing diagram202, corresponding to the 180° precession period being provided alongthe −Z axis, and thus orthogonal to the 0° precession period. However,because the precession of the magnetic moment vector B_(MM) is conical,and not planar in the example of FIG. 4 based on the external magneticfield B_(EXT) being offset of orthogonal with respect to the opticalaxis of the optical probe beam OPT_(PRB) by θ₁=45°, the externalmagnetic field B_(EXT), as observed by the optical detector(s) 28 viathe detection beam(s) OPT_(DET), is asymmetrical about zero at each 180°interval of the period of the precession. Because the precession of thealkali metal vapor is non-planar, the time at which the detection beamOPT_(DET) provides approximately equal optical energy on thephotodetectors 116 and 118 of the optical detector(s) 112 is not alignedwith the times T₁ and T₃, as opposed to in the timing diagram 200.Furthermore, in the timing diagram 202, the time at which the detectionbeam OPT_(DET) provides approximately maximum sensitivity to magneticfield gradients at a time T₅ between the time T₁ and the time T₂, andagain at a time T₆ between the times T₂ and T₃. In the example of FIG.5, the time T₅ occurs just subsequent to the time T₁, and the time T₆occurs just preceding the time T₃. Therefore, the times T₅ and T₆ arenot 180° out-of-phase of each other in the precession period of thealkali metal vapor.

As an example, the detection system 26 can provide the optical probebeam pulses OPT_(PRB) at the appropriate times at which the opticalenergy of the detection beam OPT_(DET) provided on the first and secondphotodetectors 116 and 118 is approximately equal based on the timingsignal TMR. In response to determining that the optical energy of thedetection beam OPT_(DET) provided on the first and second photodetectors116 and 118 is unequal, the detection beam OPT_(DET) can determine thatthe angle or the amplitude of the external magnetic field B_(EXT)through the sensor cell 14 is changing. As a result, the timingcontroller 30 can change the timing signal TMR to modify the times atwhich the laser system 12 provides the optical probe beam OPT_(PRB)pulses to measure the angle of the external magnetic field B_(EXT), suchas to set the detection beam OPT_(DET) to have approximately equaloptical energy of the detection beam OPT_(DET) provided on the first andsecond photodetectors 116 and 118. Accordingly, in this manner, thedetection system 26 can monitor the angle and frequency of the externalmagnetic field B_(EXT) in a feedback manner.

Therefore, as described herein, the magnitude of the external magneticfield B_(EXT) is determined by the period of the precession of thealkali metal vapor and the angle of the external magnetic field B_(EXT)is determined by the relative timing of orthogonality of the spinprojection of the magnetic moment vector B_(MM) along the optical probebeam OPT_(PRB), and thus the zero-crossings of the solid lines duringthe respective period as demonstrated in the example of FIG. 5. Thelaser system 12 can thus provide measurement pulses of the probe beamOPT_(PRB) at respective times when the zero-crossings are expected tooccur by the detection system 26 based on monitoring the timing of thezero-crossings during previous measurements. As an example, themeasurement pulses of the optical probe beam OPT_(PRB) can be providedat a much higher optical power than the calibration pulses to obtain ahighly sensitive measurement of whether the timing of the zero-crossingshas deviated from that of the previously monitored measurements, suchthat corrections can be provided accordingly by the detection system 26(e.g., via the timing controller 30).

As an example, if the magnitude of the external magnetic field B_(EXT)has changed, more optical power of the respective detection beamOPT_(DET) is observed on either the first photodiode 116 or the secondphotodiode 118, depending on whether the external magnetic field B_(EXT)has increased or decreased, during both measurement pulses in eachperiod of the precession of the alkali metal vapor. As a result, theprecession period of the alkali metal vapor is perceived by thedetection system 26 as having changed, thus indicating that the timingcontroller 30 can change the timing signal TMR to adjust the period ofthe pulse repetition of providing both the optical pump beam OPT_(PMP)and the optical probe beam OPT_(PRB) accordingly (e.g., more frequentfor an increase in amplitude or less frequent for a decrease inamplitude).

As another example, if the angle of the external magnetic field B_(EXT)has changed, the detection system 26 will observe more optical power ofthe detection beam OPT_(DET) on one of the first and second photodiodes116 and 118 during the first measurement pulse in each period and moreoptical power on the other of the first and second photodiodes 116 and118 during the second measurement pulse. In response to the detectionsystem 26 detecting the opposing disparity of optical power of thedetection beam OPT_(DET) on the first and second photodiodes 116 and 118in the two measurement pulses, the detection system 26 can command thetiming controller 30 to change the timing signal TMR to move the timingof the measurement pulses either closer together in time (e.g., if theangle is getting closer to 45°) or farther apart in time (e.g., if theangle is getting closer to 90°).

As an example, at an angle of approximately 45°, the laser system 12 canbe commanded via the timing signal TMR to provide a singleangle-measurement pulse, occurring at the time T₂. Additionally,measurement pulses of the optical probe beam OPT_(PRB) can be providedduring the times of maximum sensitivity to magnetic gradients,corresponding to times T₅ and T₆ in the timing diagram 202, or at timesT₁ and T₃ in the timing diagram 200. At any angle of the externalmagnetic field B_(EXT) between approximately 45° and 90°, up to sixoptical probe beam OPT_(PRB) pulses can be provided by the laser system12 per precession period. For example, one optical probe beam OPT_(PRB)pulse can be provided at each of the times T₀/T₄ and the time T₂, oneoptical probe beam OPT_(PRB) pulse can be provided at each zero-crossingof the spin projection of the magnetic moment vector B_(MM) along theoptical probe beam OPT_(PRB) (i.e., the solid line), and one opticalprobe beam OPT_(PRB) pulse can be provided at each magnitude maximum ofthe dotted line in the example of FIG. 5.

Thus far, the magnetometer system 10 has been described with respect todetermining a scalar value of the amplitude and frequency of theexternal magnetic field B_(EXT) based on a measurement of the opticaldetection beam OPT_(DET) through a single measurement zone (e.g., thefirst measurement zone 20) of the sensor cell 14. For example, themeasurement of the optical detection beam OPT_(DET) through the singlemeasurement zone (e.g., the first measurement zone 20) of the sensorcell 14 can facilitate determining the precession of the alkali metalvapor for generating the timing signal TMR via the timing controller 30,and the measurement of the scalar amplitude and frequency of theexternal magnetic field B_(EXT) based on the determined precession ofthe alkali metal vapor. However, as described in greater detail herein,the magnetometer system 10 can implement the second and thirdmeasurement zones 22 and 24 to determine the magnetic field gradientB_(GRDT) of the external magnetic field B_(EXT).

FIG. 6 illustrates another example of a magnetometer system 250. Themagnetometer system 250 can be implemented in any of a variety ofapplications to measure a magnetic field, such as navigation. Forexample, the magnetometer system 250 can be implemented in an inertialnavigation system (INS) for an aircraft or a spacecraft to assist withreal-time navigation or location determination.

The magnetometer system 250 includes a first pump laser 252 and a probelaser 254 that can collectively be part of the laser system 12 in theexample of FIG. 1. The first pump laser 252 is configured to generate anoptical pump beam OPT_(PMP1), and the probe laser 254 is configured togenerate an optical probe beam OPT_(PRB). The first optical pump beamOPT_(PMP1) and the optical probe beam OPT_(PRB) are combined via a beamcombiner 256. As an example, the beam combiner 256 can be configured asa 2×2 optical combiner to provide power efficient optical coupling(e.g., as opposed to a 2×1 optical combiner that can exhibit a 3 dBloss). The beam combiner 256 is demonstrated as providing a combinedbeam axis, demonstrated in the example of FIG. 6 asOPT_(PMP1)/OPT_(PRB). The combined beam axis OPT_(PMP1)/OPT_(PRB) cancorrespond to a coaxial combination of the first optical pump beamOPT_(PMP1) and the optical probe beam OPT_(PRB). It is to be understoodthat the first optical pump beam OPT_(PMP1) and the optical probe beamOPT_(PRB) are not necessarily concurrently provided together as thecombined beam axis OPT_(PMP1)/OPT_(PRB), but merely share an opticalaxis.

The combined beam axis, demonstrated in the example of FIG. 6 asOPT_(PMP1)/OPT_(PRB), is provided through a sensor cell 258 thatincludes an alkali metal vapor disposed therein. In the example of FIG.6, the sensor cell 258 includes a first measurement zone 260 (“ZONE 1”),a second measurement zone 262 (“ZONE 2”), and a third measurement zone264 (“ZONE 3”) that can each correspond to three-dimensional spatialregions within the volume of the sensor cell 258. In the example of FIG.6, the first measurement zone 260 is arranged approximately centrallyalong a length of the sensor cell 258, and the second and thirdmeasurement zones 262 and 264 are arranged at opposing ends of thesensor cell 258. The combined beam axis OPT_(PMP1)/OPT_(PRB) isdemonstrated in the example of FIG. 6 as being provided through thefirst measurement zone 260 and the second measurement zone 262 via thebeam coupler 256.

As described previously, the first optical pump beam OPT_(PMP1) can beprovided through the first and second measurement zones 260 and 262 tofacilitate precession of the alkali metal vapor in the first and secondmeasurement zones 260 and 262 in response to the external magneticfield. Therefore, the first optical pump beam OPT_(PMP1) can align themagnetic moment of the alkali metal vapor in an approximately parallelmanner with respect to the first optical pump beam OPT_(PMP1).Therefore, the alkali metal vapor can precess about the externalmagnetic field based on the alignment of the magnetic moment of thealkali metal vapor, as described with reference to the examples of FIGS.2-4.

In the example of FIG. 6, a dichroic mirror 266 is demonstrated on theopposite side of the first measurement zone 260 to stop the firstoptical pump beam OPT_(PMP1) but to allow a first detection beamOPT_(DET1) corresponding to the optical probe beam OPT_(PRB) passingthrough the first measurement zone 260 to pass to a first opticaldetector 268. The first optical detector 268 is configured to detect theFaraday rotation of the optical probe beam OPT_(PRB) through the firstmeasurement zone 260 based on the first detection beam OPT_(DET1). Thefirst optical detector 268 can provide a first detection signal DET₁ toa detection processor 270 that can correspond to a processor of thedetection system 26 in the example of FIG. 1. In response to the firstdetection signal DET₁, the detection processor 270 can generate thescalar magnetic field B_(SCLR) corresponding to the amplitude and angleof the external magnetic field B_(EXT), as described previously in theexamples of FIGS. 2-6.

In addition, the detection processor 270 can generate and adjust atiming reference TIME that is provided to a timing controller 272 thatcan correspond to the timing controller 30 in the example of FIG. 1. Thetiming controller 272 can generate a timing signal TMR_(PMP) that isprovided to the first pump laser 252 to indicate the timing ofactivation of the pulses of the first optical pump beam OPT_(PMP1),similar to as described previously. In the example of FIG. 6, themagnetometer system 250 also includes a second pump laser 274 that isconfigured to generate a second optical pump beam OPT_(PMP2) that isprovided through the third measurement zone 264, as described in greaterdetail herein. The timing signal TMR_(PMP) is thus also provided to thesecond pump laser 274 to indicate the timing of activation of the pulsesof the second optical pump beam OPT_(PMP2) concurrently with the pulsesof the first optical pump beam OPT_(PMP1). Similarly, the timingcontroller 272 can generate a timing signal TMR_(PRB) that is providedto the probe laser 254 to indicate the timing of activation of thepulses of the optical probe beam OPT_(PRB).

In addition, in the example of FIG. 6, the first optical pump beamOPT_(PRB1) and a second detection beam OPT_(DET2) corresponding to theoptical probe beam OPT_(PRB) passing through the second measurement zone262 are demonstrated as a combined beam axis. The first optical pumpbeam OPT_(PMP1) and the second detection beam OPT_(DET2) are provided toa first mirror 276 to reflect the first optical pump beam OPT_(PMP1) andthe second detection beam OPT_(DET2) to a dichroic mirror 278. Thedichroic mirror 278 blocks the first pump beam OPT_(PMP1) to provide thesecond detection beam OPT_(DET2) through another dichroic mirror 280 toreflect from another mirror 282 and through the third measurement zone264.

As described previously, the second optical pump beam OPT_(PMP2) isprovided through the third measurement zone 264. In the example of FIG.6, the magnetometer system 250 includes a dichroic mirror 284 that isconfigured to reflect the second optical pump beam OPT_(PMP2) andtransmit a third detection beam OPT_(DET3) as part of a combined beamaxis that includes a third detection beam OPT_(DET3) corresponding tothe second detection beam OPT_(DET2) passing through the thirdmeasurement zone 264 in the opposite direction as the second opticalpump beam OPT_(PMP2). In the example of FIG. 6, the second optical pumpbeam OPT_(PMP2) and the second detection beam OPT_(DET2) occupy acombined beam axis along opposite directions between the mirror 282 andthe third measurement zone 264. The dichroic mirror 280 blocks thesecond optical pump beam OPT_(PMP2).

The second detection beam OPT_(DET2) experiences a Faraday rotationthrough the third measurement zone 264, which is exhibited in the thirddetection beam OPT_(DET3). In the absence of a magnetic field gradient,the Faraday rotation is approximately identical to the Faraday rotationof the optical probe beam OPT_(PRB) as it passes through the secondmeasurement zone 262. However, because the second detection beamOPT_(DET2) passes through the third measurement zone 264 after havingundergone a net 180° reflection after the optical probe beam OPT_(PRB)has passed through the second measurement zone 262, the Faraday rotationexperienced by the second detection beam OPT_(DET2) has been reflected;that is, it enters the third detection zone 264 in a direction that isopposite the Faraday rotation experienced by the optical probe beamOPT_(PRB). Therefore, the third detection beam OPT_(DET3) thus exhibitsthe Faraday rotation experienced by the second detection beamOPT_(DET2), similar to the second detection beam OPT_(DET2) exhibitingthe Faraday rotation experienced by the optical probe beam OPT_(PRB), inthe same rotation direction. The third detection beam OPT_(DET3) isprovided through to a second optical detector 286. The second opticaldetector 286 is configured to detect the Faraday rotation of the seconddetection beam OPT_(DET2) through the third measurement zone 264 basedon the third detection beam OPT_(DET3), and thus determines a differencein Faraday rotation between the second and third measurement zones 262and 264, as described in greater detail herein. The second opticaldetector 286 can provide a second detection signal DET₂ to the detectionprocessor 270 to determine the magnetic field gradient B_(GRDT), asdescribed in greater detail herein.

As an example, if the conditions of the alkali metal vapor in each ofthe second and third measurement zones 262 and 264 are approximately thesame, then the Faraday rotation of the optical probe beam OPT_(PRB)through the second measurement zone 262, as provided by the seconddetection beam OPT_(DET2) and after reflection from mirrors 276 and 282,will be approximately equal and opposite the Faraday rotation of thesecond detection beam OPT_(DET2) through the third measurement zone 264,as provided by the third detection beam OPT_(DET3). As an example, theconditions can include the density of the alkali metal vapor,temperature of the sensor cell 258, or other varying calibrationconditions differ between the second and third measurement zones 262 and264. As another example, if the amplitude of the external magnetic fieldB_(EXT) is the same in the second and third measurement zones 262 and264 (e.g., there is no magnetic field gradient of the external magneticfield B_(EXT)), then similarly, the Faraday rotation of the opticalprobe beam OPT_(PRB) through the second measurement zone 262, asprovided by the second detection beam OPT_(DET2), will after reflectingfrom mirrors 276 and 282 be approximately equal and opposite the Faradayrotation of the second detection beam OPT_(DET2) through the thirdmeasurement zone 264, as provided by the third detection beamOPT_(DET3). However, variations in the conditions of the alkali metalvapor in each of the second and third measurement zones 262 and 264 canresult in a different amount of Faraday rotation.

As a result, upon the detection processor 270 determining that the thirddetection beam OPT_(DET3) has a non-zero Faraday rotation via the secondoptical detector 286, then the detection processor 270 can determinethat either the conditions of the alkali metal vapor, as describedpreviously, are different, or a magnetic field gradient of the externalmagnetic field B_(EXT) exists. For example, the detection processor 270can determine if there is a non-zero Faraday rotation of the seconddetection beam OPT_(DET2) based on the third detection beam OPT_(DET3)during the calibration pulses of the optical probe beam OPT_(PRB), suchas provided in the first and second states 104 and 106 described in theexample of FIG. 3.

If a non-zero Faraday rotation of the second detection beam OPT_(DET2)based on the third detection beam OPT_(DET3) is detected during thecalibration pulses of the optical probe beam OPT_(PRB), then thedetection processor 270 can determine that the conditions of the alkalimetal vapor (e.g., density, population, and/or temperature) aredifferent between the second and third measurement zones 262 and 264. Inresponse, the detection processor 270 can adjust the power of the secondpump laser 274, such that the second optical pump beam OPT_(PMP2) cancompensate for the different conditions of the alkali metal vapor in afeedback manner. If the detection processor 270 determines that there isno (e.g., net zero) Faraday rotation of the second detection beamOPT_(DET2) based on the third detection beam OPT_(DET3) is detectedduring the calibration pulses of the optical probe beam OPT_(PRB), thenthe detection processor 270 can determine that the conditions of thealkali metal vapor (e.g., density, population, and/or temperature)between the second and third measurement zones 262 and 264 areapproximately the same, and thus that the magnetometer system 10 iscalibrated with respect to the conditions of the alkali metal vapor inthe sensor cell 258. However, if the detection processor 270 determinesthat there is no (e.g., net zero) Faraday rotation of the seconddetection beam OPT_(DET2) based on the third detection beam OPT_(DET3)is detected during the calibration pulses of the optical probe beamOPT_(PRB), but determines that there is a non-zero Faraday rotation ofthe second detection beam OPT_(DET2) based on the third detection beamOPT_(DET3) during the interrogation pulses (e.g., during the third state108 in the example of FIG. 3, such as corresponding to the times T₅ andT₆ in the example of FIG. 5), then the detection processor 270 candetermine that there is a magnetic field gradient in the externalmagnetic field B_(EXT). Accordingly, the detection processor 270 canmeasure the magnetic field gradient B_(GRDT) based on the detectedFaraday rotation via the third detection beam OPT_(DET3).

As an example, the magnetometer system 250 can provide optical pumpingand detection through multiple axes of the sensor cell 258 to providegreater precision and vector amplitude detection of the externalmagnetic field B_(EXT). For example, the magnetometer system 250 caninclude optics or additional pump and probe lasers to provide opticalpump beam(s) and optical probe beam(s) through at least one otherorthogonal axis of the sensor cell 258 to provide additionalmeasurements of the external magnetic field B_(EXT). Accordingly, themagnetometer system 250 can measure the amplitude, vector angles, andmagnetic field gradients associated with the external magnetic fieldB_(EXT) based on providing pump and probe beam pulses through multipleorthogonal directions through the sensor cell 258.

In view of the foregoing structural and functional features describedabove, a methodology in accordance with various aspects of the presentinvention will be better appreciated with reference to FIG. 7. While,for purposes of simplicity of explanation, the methodology of FIG. 7 isshown and described as executing serially, it is to be understood andappreciated that the present invention is not limited by the illustratedorder, as some aspects could, in accordance with the present invention,occur in different orders and/or concurrently with other aspects fromthat shown and described herein. Moreover, not all illustrated featuresmay be required to implement a methodology in accordance with an aspectof the present invention.

FIG. 7 illustrates an example of a method 300 for measuring an externalmagnetic field (e.g., the external magnetic field B_(EXT)). At 302, acircularly-polarized optical pump beam (e.g., the optical pump beamOPT_(PMP)) is generated via a pump laser (e.g., the pump laser(s) 14).At 304, a linearly-polarized optical probe beam (e.g., the optical probebeam OPT_(PRB)) is generated via a probe laser (e.g., the probe laser(s)16). At 306, the circularly-polarized optical pump beam is providedthrough a sensor cell (e.g., the sensor cell 18) comprising alkali metalvapor in a pulsed-manner based on a timing signal (e.g., the timingsignal TMR) to facilitate precession of the alkali metal vapor inresponse to the external magnetic field. At 308, the linearly-polarizedoptical probe beam is provided through the sensor cell in apulsed-manner based on the timing signal to provide a detection beam(e.g., the detection beam OPT_(DET)) corresponding to thelinearly-polarized optical probe beam exiting the sensor cell. At 310,the precession of the alkali metal vapor is detected based on thedetection beam. At 312, the timing signal is generated based on thedetected precession of the alkali metal vapor. At 314, an amplitude anddirection of the external magnetic field are calculated based on thedetected precession of the alkali metal vapor.

What have been described above are examples of the present invention. Itis, of course, not possible to describe every conceivable combination ofcomponents or methodologies for purposes of describing the presentinvention, but one of ordinary skill in the art will recognize that manyfurther combinations and permutations of the present invention arepossible. Accordingly, the present invention is intended to embrace allsuch alterations, modifications and variations that fall within thespirit and scope of the appended claims. Additionally, where thedisclosure or claims recite “a,” “an,” “a first,” or “another” element,or the equivalent thereof, it should be interpreted to include one ormore than one such element, neither requiring nor excluding two or moresuch elements. As used herein, the term “includes” means includes butnot limited to, and the term “including” means including but not limitedto. The term “based on” means based at least in part on.

What is claimed is:
 1. A magnetometer system comprising: a sensor cellcomprising alkali metal vapor; a laser system configured to provide anoptical pump beam through the sensor cell in a pulsed manner tofacilitate precession of the alkali metal vapor in response to anexternal magnetic field and to provide an optical probe beam through thesensor cell in a pulsed manner based on a precession frequency of thealkali metal vapor; and a detection system configured to detect theprecession of the alkali metal vapor in response to a detection beamcorresponding to the optical probe beam exiting the sensor cell and tocalculate an amplitude and direction of the external magnetic fieldbased on the detected precession of the alkali metal vapor.
 2. Thesystem of claim 1, wherein the sensor cell comprises a first measurementzone, a second measurement zone, and a third measurement zone, whereinthe second and third measurement zones are arranged at opposite ends ofthe sensor cell, wherein the detection system is configured to detectthe precession of the alkali metal vapor in response to a firstdetection beam, a second detection beam, and a third detection beamcorresponding to the optical probe beam exiting the sensor cell througheach of the respective first, second, and third measurement zones. 3.The system of claim 2, wherein the optical pump beam is provided througheach of the first, second, and third measurement zones, wherein theoptical probe beam is provided through the first measurement zone in thepulsed manner to calculate the amplitude and direction of the externalmagnetic field, wherein the optical probe beam is provided through eachof the second and third measurement zones to calculate a magnetic fieldgradient across the sensor cell.
 4. The system of claim 2, wherein theoptical probe beam is provided through the second measurement zone togenerate the second detection beam, wherein the second detection beam isreflected through the third measurement zone to generate the thirddetection beam, such that the third detection beam is associated withthe precession of the alkali metal vapor in each of the second and thirdmeasurement zones.
 5. The system of claim 4, wherein the laser systemcomprises a 2×2 optical coupler configured to axially combine theoptical pump beam and the optical probe beam as a first combined beamaxis through the first measurement zone and a second combined beam axisthrough the second measurement zone.
 6. The system of claim 2, whereinthe laser system comprises: a first pump laser configured to generate afirst optical pump beam that is provided through each of the first andsecond measurement zones in a pulsed manner to facilitate precession ofthe alkali metal vapor in each of the first and second measurement zonesin response to the external magnetic field; and a second pump laserconfigured to generate a second optical pump beam that is providedthrough the third measurement zone to facilitate precession of thealkali metal vapor in the third measurement zone in response to theexternal magnetic field.
 7. The system of claim 6, wherein the lasersystem further comprises a probe laser configured to generate theoptical probe beam, wherein the optical probe beam is configured toprovide a calibration optical probe beam concurrently with each of thefirst and second optical pump beams, wherein the laser system is furtherconfigured to calibrate the second pump laser in response to adifference between the second detection beam and the third detectionbeam.
 8. The system of claim 7, wherein the optical probe beam isconfigured to provide a startup beam at an initial startup of themagnetometer system to provide an initial detection beam, wherein thedetection system is configured to generate an initial estimate of theprecession frequency of the alkali metal vapor and to determine aninitial amplitude and direction of the external magnetic field based onthe initial estimate of the precession frequency.
 9. The system of claim1, wherein the detection system comprises a timing controller configuredto generate a timing signal based on the detected precession of thealkali metal vapor in response to the detection beam, wherein the timingcontroller is configured to control timing of application of pulsesassociated with the optical pump beam and to control timing ofapplication of pulses associated with the optical probe beam based onthe timing signal to track changes in the amplitude and/or direction ofthe external magnetic field based on the detection beam.
 10. The systemof claim 1, wherein the optical pump beam and the optical probe beamhave a common optical axis, wherein the detection system is configuredto command the laser system to provide an optical pump beam pulse inresponse to a magnetic moment of the alkali metal vapor beingapproximately parallel with the common optical axis based on thedetected precession of the alkali metal vapor, and is further configuredto command the laser system to provide an optical probe beam pulse inresponse to the magnetic moment of the alkali metal vapor beingapproximately orthogonal with the common optical axis based on thedetected precession of the alkali metal vapor to monitor the precessionof the alkali metal vapor.
 11. The system of claim 10, wherein thedetection system is further configured to command the laser system toprovide an optical probe beam pulse in response to a magnetic moment ofthe alkali metal vapor being each of approximately parallel with thecommon optical axis and approximately anti-parallel with the commonoptical axis based on the detected precession of the alkali metal vaporto calibrate the magnetometer system.
 12. A method for measuring anexternal magnetic field via a magnetometer system, the methodcomprising: generating a circularly-polarized optical pump beam via apump laser; generating a linearly-polarized optical probe beam via aprobe laser; providing the circularly-polarized optical pump beamthrough a sensor cell comprising alkali metal vapor in a pulsed-mannerbased on a timing signal to facilitate precession of the alkali metalvapor in response to the external magnetic field; providing thelinearly-polarized optical probe beam through the sensor cell in apulsed-manner based on the timing signal to provide a detection beamcorresponding to the linearly-polarized optical probe beam exiting thesensor cell; detecting the precession of the alkali metal vapor based onthe detection beam; generating the timing signal based on the detectedprecession of the alkali metal vapor; and calculating an amplitude anddirection of the external magnetic field via a detection processor basedon the detected precession of the alkali metal vapor, the detectionprocessor providing the amplitude and direction of the external magneticfield as an output.
 13. The method of claim 12, wherein providing thecircularly-polarized optical pump beam through the sensor cell andproviding the linearly-polarized optical probe beam through the sensorcell comprises providing each of the circularly-polarized optical pumpbeam and the linearly-polarized optical probe beam through each of afirst measurement zone, a second measurement zone, and a thirdmeasurement zone, wherein the second and third measurement zones arearranged at opposite ends of the sensor cell, wherein detecting theprecession of the alkali metal vapor comprises detecting the precessionof the alkali metal vapor in response to a first detection beam, asecond detection beam, and a third detection beam corresponding to theoptical probe beam exiting the sensor cell through each of therespective first, second, and third measurement zones.
 14. The method ofclaim 13, wherein calculating the amplitude and direction of theexternal magnetic field via the detection processor comprisescalculating a scalar magnitude of the amplitude and direction of theexternal magnetic field in response to the first detection beam andcalculating a magnetic field gradient in response to the second andthird detection beams, wherein the output of the detection processorcomprises the scalar magnitude of the amplitude of the external magneticfield, the direction of the external magnetic field, and the magneticfield gradient.
 15. The method of claim 13, wherein providing thelinearly-polarized optical probe beam through the sensor cell comprises:providing the linearly-polarized optical probe beam through the firstmeasurement zone to generate the first detection beam; and providing thelinearly polarized optical probe beam through the second measurementzone to generate the second detection beam; and reflecting the seconddetection beam through the third measurement zone via optics to generatethe third detection beam, such that the third detection beam isassociated with the precession of the alkali metal vapor in each of thesecond and third measurement zones.
 16. The method of claim 12, whereinproviding the linearly-polarized optical probe beam comprises providingthe linearly-polarized optical probe beam through the sensor cell inresponse to a magnetic moment of the alkali metal vapor being each ofapproximately parallel with the common optical axis and after themagnetic moment of the alkali vapor has precessed approximately 180°based on the detected precession of the alkali metal vapor to calibratethe magnetometer system.
 17. A magnetometer system comprising: a sensorcell comprising alkali metal vapor and comprising a first measurementzone, a second measurement zone, and a third measurement zone, whereinthe second and third measurement zones are arranged at opposite ends ofthe sensor cell; a laser system configured to provide an optical pumpbeam through each of the first, second, and third measurement zones ofthe sensor cell in a pulsed manner to facilitate precession of thealkali metal vapor in response to an external magnetic field and toprovide an optical probe beam through each of the first, second, andthird measurement zones of the sensor cell in a pulsed manner based on aprecession frequency of the alkali metal vapor; and a detection systemconfigured to detect the precession of the alkali metal vapor inresponse to a first detection beam, a second detection beam, and a thirddetection beam corresponding to the optical probe beam exiting thesensor cell through the first, second, and third measurement zones,respectively, to calculate a scalar amplitude and direction of theexternal magnetic field in response to the first detection beam, and tocalculate a magnetic field gradient of the external magnetic field inresponse to the second and third detection beams.
 18. The system ofclaim 17, wherein the optical probe beam is provided through the secondmeasurement zone to generate the second detection beam, wherein thesecond detection beam is reflected through the third measurement zone togenerate the third detection beam, such that the third detection beam isassociated with the precession of the alkali metal vapor in each of thesecond and third measurement zones.
 19. The system of claim 17, whereinthe detection system comprises a timing controller configured togenerate a timing signal based on the detected precession of the alkalimetal vapor in response to the first detection beam, wherein the timingcontroller is configured to control timing of application of pulsesassociated with the optical pump beam and to control timing ofapplication of pulses associated with the optical probe beam based onthe timing signal to track changes in the amplitude and/or direction ofthe external magnetic field based on the detection beam.
 20. The systemof claim 17, wherein the optical pump beam and the optical probe beamhave a common optical axis, wherein the detection system is furtherconfigured to command the laser system to provide an optical probe beampulse in response to a magnetic moment of the alkali metal vapor beingeach of approximately parallel with the common optical axis and afterthe magnetic moment of the alkali vapor has precessed by approximately180° based on the detected precession of the alkali metal vapor tocalibrate the magnetometer system.